* print.c: Added code for detection of circular references during

* print.c: Added code for detection of circular references during
printing.  (init_ref_stack, grow_ref_stack): New functions.  Added
a hook for printing of closures (accessible via print options).
This leads to a split of calls to scm_iprin1 into two classes:
elementary print operations (e. g. the code which prints a smob)
still use scm_iprin1 while top level calls (like scm_display) use
scm_prin1.  scm_prin1 begins by clearing the data structure used
to record reference information.